On a stock 1956 PA18A 150 Super Cub would if have a heavy case engine or wide case engine (not sure of the differences in the engines? Desireable or need to change?
Are you asking about a wide vs narrow deck engine? If so I believe a 1956 0320 150 hp would be a narrow deck. The big question is that the motor that came with the plane? Someone may have changed it in the last 63 years. It does not matter what the logs/owner/you buddy says, go look at the data tag on the motor and see what it says.
DENNY
If it has the original engine it would be a narrow deck. Nothing wrong with a narrow deck. If is on a plane in Kalispel, I think it has an O-320-A2B. I think that engine could be either wd or nd.